Electrical devices such as transformers, reactors, capacitors, circuit breakers, among others, use dielectric fluids inside them, also called dielectric oils, which are used as a medium for insulation and dissipation of heat generated by the internal components of the device.
The dielectric fluid must be able to effectively and reliably perform its functions as a cooling and isolation medium during the service life of the electrical device.
Obviously, many of the properties of the dielectric fluid should be considered as necessary to provide the expected dielectric and cooling capacities, according to its application, so that depending thereon, their ability to function effectively and reliably can be affected. These properties may include: dielectric strength, dielectric constant, dissipation factor, viscosity, acid number, pour temperature, and ignition temperature.
The dielectric strength of the fluid indicates its ability to resist electrical breakdown at certain power frequencies and is measured as the minimum electrical voltage required to cause arcing between two electrodes immersed in the dielectric fluid.
The dielectric constant is the ratio of the capacitance of a capacitor containing a dielectric (oil) between its plates, between the capacitance of the same capacitor when the dielectric is a vacuum. This property is related to the ability of the dielectric fluid to conduct electricity, so that the lower the value thereof the higher capacity shall the dielectric fluid have.
The dissipation factor of a dielectric fluid is the measure of the dielectric losses in that fluid, generally as heat. A low dissipation factor indicates low dielectric losses and a low concentration of polar contaminants soluble in the dielectric fluid.
The acid number in a dielectric fluid is a measure of the constituents or acid pollutants of the fluid. The acidity of a dielectric fluid is due to the formation of acid oxidation products. Acids and other oxidation products, along with water and solid contaminants, will affect the dielectric properties and others of the dielectric fluid. The increase rate of the dielectric fluid acid number is a good indicator of the aging rate thereof.
Pour temperature is the temperature at which a fluid stops flowing, taking the solid state at ambient pressure. The pour temperature may also be defined as the maximum temperature (usually below zero ° C.) at which the dielectric fluid can not move or deform under its own weight. The lower this temperature, the dielectric fluid shall be applied in electrical devices subject to extreme temperatures below zero ° C.
The ignition temperature, also known as flash point, is the temperature that the dielectric fluid should reach for resulting in the ignition of the vapors thereof, when exposed to air and the source of ignition.
Since dielectric fluids cool by convection the electric devices to which they are applied, the viscosity of these fluids at various temperatures is another important factor to consider. Viscosity is a measure of the resistance of a fluid to flow and is typically analyzed in terms of kinematic viscosity. At lower viscosities, the dielectric fluid circulates or flows better inside the electrical device and thus allows better heat dissipation.
Among the dielectric fluids used in electrical devices and that have these and other properties, those dielectric fluids based on mineral oils, silicones, synthetic oils, vegetable oils with antioxidants or mixtures thereof are included.
Dielectric fluids based on mineral oils derived from petroleum, silicone based oils or synthetic oils have been widely used in electrical transformers, power cables and capacitors. Examples of these oils are found in U.S. Pat. Nos. 4,082,866, 4,206,066, 4,621,302, 5,017,733, 5,250,750 and 5,336,847.
It has been shown that the incorporation of nanoparticles to dielectric fluids or lubricants based on mineral oil improves the tribological properties thereof, as described by the following patent documents.
Aruna Zhamu et al., in published US patent application US-2011/0046027 A1, discloses a lubricant composition based on mineral oil or synthetic oil including graphene nanoflakes in a dispersed form and in an amount of 0.001% to 60% by weight. These are preferably single layer graphene nanoflakes, and depending on their amount, the lubricant tends to be a fat. Mineral oil or synthetic oil modified with these graphene nanoflakes has a better thermal conductivity, a better friction coefficient, improved anti-wear performance and better viscosity stability compared to mineral oils or synthetic oils modified only with graphene nanoparticles or carbon nanotubes.
Jian Li et al. from University of Chongqing in Chinese patent application publication CN101950601, disclose a dielectric mineral oil used in power transformers, including nanoparticles which are dispersed in the mineral oil by ultrasound. The incorporation of nanoparticles in the dielectric mineral oil improves the dielectric strength thereof as compared to dielectric mineral oils without nanoparticles added.
However, in both techniques indicated above the synergistic use of the benefits of incorporating in a dielectric mineral oil a combination of graphene nanoflakes and nanoparticles is not described. Therefore, the present invention provides a dielectric mineral oil having excellent thermal conductivity and stability resulting from the synergistic incorporation of graphene nanoflakes and nanoparticles, whether metal nanoparticles, ceramic nanoparticles and combinations thereof.